The present invention relates to a testing apparatus for determining the mechanical properties of viscoelastic materials and more particularly to a testing apparatus for determining the stress-strain characteristics of energetic materials such as solid propellants and plastic bonded explosives under high deformation rates up to approximately 40,000 inches per minute.
The mechanical properties of solid propellants are typically measured with an Instron tensile tester at extension rates up to 50 inches per minute or with hydraulic tensile tester for rates up to 10,000 inches per minute. In order to predict the mechanical properties of such energetic materials at higher extension rates the time-temperature superposition principle for linear viscoelastic materials is applied. To correctly apply the time-temperature superposition principle, the tensile properties of the energetic material are measured at lower strain rates at various temperatures. The Williams-Landel-Ferry (WLF) shift procedure, which is commonly known to those persons skilled in this art, is then applied to obtain a reduced master curve of the estimated tensile properties at higher extension rates at the referenced temperature. Highly filled polymers, such as solid propellants, however, often display non-linear viscoelastic behavior. Thus, the shape of the stress-strain curve at very high deformation rates or the tensile strain and energy at the break point cannot be accurately predicted using WLF shifts.
The uniaxial high extension rate properties of solid propellants and plastic bonded explosives have become important for characterizing the damage and energy absorption behavior of these materials during operational use. Damage behavior is often characterized by the presence of voids in the propellants. Since solid propellants are highly filled polymers, any finite deformation will inevitably cause high stress fields proximate the filler particles which leads to interfacial debonding, often called dewetting. Dewetting causes the formation of voids and a corresponding increase in propellant volume which can be measured with a dilatometer. At high strain rates, there is a pronounced increase in voids produced in the propellant relative to slow strain rates. Similarly, the void collapse after specimen failure is typically very fast but is also dependent on strain rate. These void formation and collapse variations are primarily due to the viscoelastic material behavior. Further study of this viscoelastic material behavior can be accomplished with the aid of a high extension rate tensile testing apparatus.
In addition, a change in the fracture mode of the propellants at high deformation rates has recently been observed and documented with the use of high speed photography. The phenomena of multiple fracture of solid propellants seems to occur at high deformation rates at ambient temperature. Above a critical stain rate, the uniaxial test specimen breaks into several pieces instead of the traditional two pieces. Empirical measurements of this viscoelastic fracture phenomena is accomplished with the use of a fully instrumented high extension rate tensile testing apparatus. Only with a high extension rate tensile testing apparatus such as the present invention can one empirically and accurately identify these critical strain rates.
Increased deformation rate testing of plasticized explosives has previously been accomplished using the Hopkinson split bar technique. See E. James and D. Breithaupy, "High Strain Rate Mechanical Testing of Solid Propellants by the Hopkinson Split Bar Technique", Lawrence Livermoore National Laboratory, UCID-20021, February, 1984. Strain rates up to 10,000 per second were obtained for impact rates up to 150,000 inches per minute. However, the small sample size typically used in Hopkinson Bar Test may give rise to significant errors.
In addition, the use of a Charpy-lzod impact pendulum for such increased deformation rate tensile testing of propellants has been attempted. See Chemical Propulsion Information Agency (CPIA) Publication 283, April 1977, pp 41-50. The straining rates attained by the Charpy-lzod impact pendulum are dependent on the length of the pendulum arm as well as the maximum release angle. Such a device is known to be suitable for extension rates up to about 12,000 inches per minute.
The use of a flywheel for impact testing of polymers is a technique that has also been previously attempted. A flywheel has certain advantages over the Hopkinson's split bar and the pendulum. The total energy losses imparted to the test specimen can easily be determined by measuring the area under the stress-strain curve or force versus extension curve. On the other hand, when employing the Hopkinson's split bar technique, the entire moving mass must be stopped after the test is completed. A significant problem with previous flywheel tensile test devices is the inability to achieve accurate tick for the engagement and disengagement of the test specimen.
The previously known flywheel type tensile testers would work in typically one of two manners. In the first arrangement, the flywheel would attain an approximate constant velocity and then the test specimen would be moved or rotated into position for engagement. The test specimen would not be engaged by the flywheel until the rotational velocity of the flywheel achieves a predetermined constant value. This test speed for a flywheel is a function of the flywheel diameter and the number of rotations per unit of time. Thus the movement or rotation of the test specimen needed to occur very quickly and synchronized with the flywheel such that the test specimen is properly engaged. For this reason, the test speed of such flywheel tensile testers has proved to be a constraining parameter because the specimen often could not properly be aligned with the engaging mechanism in sufficient time. Similar problems would be encountered if one kept the test specimen stationary and tried to move or rotate the flywheel into position for engagement.
The second alternative would be to keep the test specimen stationary but align the test specimen with the flywheel in a position suitable for engagement by the engaging claw. Since, the engaging claw is placed at a certain rotational position of the flywheel, the time available for ramping the flywheel up to a constant velocity is less than the time it normally takes for one rotation of the flywheel. Many of the flywheels used in such testing often involve a flywheel configuration having a significant mass which hindered the ability of the flywheel to attain a constant velocity prior to engaging the test specimen. Again, this situation limited the test speed of such flywheel tensile testers which in turn limits the extension rates the test specimen could be subjected to.